U.S. President Donald Trump's remark that "Korea used to be part of China" is disturbing Koreans.

Trump made the claim during an interview with The Wall Street Journal after meeting Chinese President Xi Jinping on Apr. 12.

Trump said in the interview, "(Xi) then went into the history of China and Korea. Not North Korea, Korea. And you know, you're talking about thousands of years and many wars. And Korea actually used to be a part of China."

American online news agency Quartz later published the claim. The agency reported that Trump made "a shocking admission of ignorance" by this comment and expected it would upset Koreans.

It was not confirmed whether Trump's controversial statement was initially delivered by Xi to him or was Trump's original idea.

Quartz said there were two times in history that could be misunderstood as the Korean Peninsula being part of China. Once was during the Han Dynasty (206 BC – 220 AD) when China made four "commanderies" in the northern part of the peninsula. Whether it could be seen as the Chinese ruling is still controversial among historians. The other event is from the Mongol empire's domination in the 13th century including China and Korea. But that period is also controversial because the existence of direct rule in Korea was "elusive."

"No respectable historian would make such a claim (that Korea used to be part of China)," Kyung Moon Hwang, a history professor at the University of Southern California, told Quartz.